The Last Diaries: In and Out of the Wildernessby Alan ClarkWeidenfeld & Nicholson £20, pp416

The mystery deepens. I could never understand why the first two volumes of Alan Clark's diaries enjoyed such critical and commercial success. And the praise that The Last Diaries has already received is even more difficult to explain. Clark was a moderately accomplished military historian, undistinguished junior Minister and boor. Because he was never at the centre of events, his political 'revelations' are, at best, second-hand and what some critics have called panache is little more than a childish enthusiasm for schoolboy abuse. The Last Diaries adds self-pity to the other shortcomings. Yet otherwise sensible people compare him with Pepys and Boswell.

It must be conceded that The Last Diaries does possess one virtue that its predecessors could not claim. Part of the book is beautifully written. Unfortunately, it is Ion Trewin's introduction. The elegance of Trewin's style depends, in part, on understatement. So he merely expresses 'surprise that Clark does not write more about some of the key events in which he was involved during the 1990s, among them the Matrix Churchill trial and the Scott inquiry which followed'. The arms to Iraq scandal is only one of the omissions. In fact, The Last Diaries barely mentions anything of real importance.

The entry for 4 November 1992 is typical of the whole volume. 'Testifying in the Matrix Churchill trial. Day wasted though I get £70 for an ITN interview at lunchtime on John Major leadership.' Then Clark moves on to what he regards as important. 'Yesterday I met V for a drink, not looking bad really, well preserved. V good bust and very restrainedly [sic] dressed. She said I would have been leader, "got the leadership", if I'd still been in [the House].'

Clark constantly returns to the two themes that the drink with V exemplifies, sex and the thought that he might have been (or should have been) Prime Minister. We know that V was a member of what Clark charmingly calls 'the coven', a woman and her two daughters who had been his mistress in turn. But her qualifications as a political commentator remain unknown. She is simply one of the innumerable friends, acquaintances and mistresses who are recorded as sharing Clark's view that the premiership should have been his. Since the idea is clearly ridiculous, it is hard to believe that they existed outside the imagination.

Musing about VE Day, Clark writes that, back in 1945, he 'did assume' that he would be Prime Minister - not hope or dream but 'assume'. The conceit that afflicted him in youth persisted into old age. It is reflected in the diary by the obvious belief that everything he does is worth recording. Readers are told of the frustration he feels when he cannot find his motor insurance certificate, where he changed trains on a journey between Stansted and London, and details of his dental work. The one consolation is that the trivia occupies space that might otherwise have been taken up with two other preoccupations - appearance and bodily functions.

The index of The Last Diaries refers to Clark's support for 'animal rights' - an underrated virtue of which he often boasted. However, in an unrecorded incident, he promised the Conservatives of Kensington and Chelsea that, if they chose him as their parliamentary candidate, he would defend fox hunting in the House of Commons. Charity requires us to take that as evidence of his desperation to return to active politics. That ambition was, at least, achieved. But success came too late.

The Last Diaries was completed by Jane, normally described as Alan Clark's 'long-suffering wife', after her husband died of a brain tumour. But it is clear that Clark was obsessed with his own mortality long before the fatal disease was diagnosed. It was all part of his self-obsession, which was more widely recognised than he knew.

Back in 1993, Clark and I were both guests on Brian Walden's television show. Each of us had been commissioned to write a review of Margaret Thatcher's memoirs, and the publisher, with newspaper photographs in mind, sent our review copies round to the television studio guarded by Securicor employees wearing crash helmets and carrying baseball bats.

We all turned at once to the index. Five references to Walden, one to Clark, none to me. Clark read his mention out loud. 'Every drama has an interval. Even Macbeth has the porter's scene.' On the night of her resignation she had 'a short talk with Alan Clark'. After a long pause, Clark asked, 'What on earth does it mean?' 'It means,' Walden said, 'you were pissed.' Clearly Margaret Thatcher shared my view of the diarist's importance.